The invention relates to a procedure for reactivating catalysts which comprise noble metal such as platinum supported on a refractory oxide of low acidity, particularly silica.
Supported metal catalysts are known to have a wide variety of catalytic applications, for example, in hydrogenations and dehydrogenations. A mild hydrogenation of unsaturated hydrocarbons has important uses in petroleum processing, and also in the pharmaceutical, food, cosmetic and photography industries. The properties of supported metal catalysts depend on the properties of both the metal and the support.
Noble metals are known to have catalytic activity in converting organic compounds. For example, such metals may be used in the hydrogenation and dehydrogenation of hydrocarbons. It will be noted that noble metals, such as platinum, are extremely expensive and rare. Accordingly, when these materials are used as catalysts they are generally uniformly distributed on a suitable support material. In this regard, the noble metal should be as finely dispersed as possible in order to provide a maximum surface area for contact with reactant molecules.
A number of materials have been used to support noble metal catalysts. These materials may be either essentially catalytically inactive or also possess catalytic properties which function in an additive or synergistic fashion with the catalytic properties of the noble metal. An example of an essentially catalytically inactive support material is gamma-alumina. An example of a catalytically active support material is an acidic aluminosilicate zeolite. Such zeolites have acid catalytic activity such as the cracking of hydrocarbons. Accordingly, a noble metal suitably supported on an appropriate zeolite may provide an excellent hydrocracking catalyst, wherein acid sites on the zeolite promote the cracking of hydrocarbons, while the noble metal, in close proximity to these acid sites, promotes the hydrogenation of the cracked products.
When relatively mild catalytic conditions are needed, it is desirable to use an inactive, amorphous support, such as silica, with a highly dispersed metal phase. Amorphous is intended to mean non-crystalline. The use of silica gel as a support is described, for example, in U.S. Pat. No. 3,969,274. Inactive supports do not interact with the metal to increase acid catalytic activity.
When solid support or catalytic materials are exposed to hydrocarbons for prolonged periods of time, particularly at elevated temperatures, the catalysts become deactivated due to the deposition on the catalyst of carbonaceous residues, e.g. coke. To restore activity, this carbonaceous residue must be removed. One way of removing coke is to oxidize (e.g. burn) off the hydrocarbonaceous deposit by exposing the catalyst to a source of oxygen (e.g. air), at elevated temperatures. However, the severe conditions encountered in oxidations may have a detrimental effect on certain supported noble metal catalysts. More particularly, as pointed out in U.S. Pat. No. 4,657,874, the entire disclosure of which is expressly incorporated herein by reference, when highly siliceous noble metal-containing zeolites are subjected to coke-burnoff, the noble metal thereof agglomerates, thereby substantially reducing the surface area of the noble metal. Note particularly, Example 5 of U.S. Pat. No. 4,657,874. The agglomerated noble metal on the zeolite can be redispersed by certain chemical treatments, but, as pointed out in this Example 5, this redispersion falls short of achieving the original level of high dispersion of noble metal before agglomeration.
The nature of the support material can have a profound effect on the manner in which noble metals can be distributed thereon under various conditions. As mentioned hereinabove, noble metals can be very finely distributed on highly siliceous zeolites. However, when subjected to the conditions of coke burn-off, this distribution is disturbed and agglomerates of noble metals form. The noble metal in these agglomerates can be only partially redistributed on the surface of the highly siliceous zeolites. In contrast to the surface of highly siliceous zeolites, the surface of gamma-alumina tends to more tenaceously hold noble metals. More particularly, noble metal supported on gamma-alumina will tend to agglomerate to a much less extent when such materials are subjected to the conditions of coke burn-off.
Examples of materials which have an entirely different surface chemistry than highly siliceous zeolites are three-dimensional microporous crystal framework structures consisting essentially of corner-sharing oxide tetrahedra of alumina and phosphorus. An example of such a material is termed an aluminophosphate in U.S. Pat. Nos. 4,310,440 and 4,385,994.
Various methods have been devised to redisperse agglomerated metals, generally employing a halogen compound. But the nature of the support material can have a serious effect on how effectively noble metals can be distributed and redistributed. While it is not intended to be bound by theory, the ability of a support material to inhibit the agglomeration of noble metal and to promote redispersion of noble metals appears to be a function of the surface chemistry of the support material, e.g. in terms of the charge and charge distribution. Because of a low surface free energy, supports having low acidity interact weakly with the metal. Each type of support, e.g. zeolites, AlPO.sub.4 's, alumina, has its own distinct surface chemistry. Therefore it is impossible to predicate a model for dispersion of noble metal on silica supports based on dispersion on these other supports.
Processes which utilize chlorine and oxygen in crystalline zeolite catalyst reactivation and metal redispersion are well known. For example, U.S. Pat. No. 3,986,982 to Crowson et al. treats deactivated platinum group metal-loaded zeolites by contacting them with a stream of inert gas containing from 0.5 to 20 vol. % of free oxygen, and from 5 to 500 ppm volume of chlorine as molecular chlorine, HCl or inorganic chlorine-containing material. The resulting catalyst is purged to remove residual oxygen and chlorine and then reduced in a stream of hydrogen at 20020 -600.degree. C.
U.S. Pat. No. 4,657,874 describes redispersing agglomerated noble metal on a high silica zeolite by contacting with inert gas containing chlorine at a partial pressure of about to about 15 Torr and water in a concentration in which the ratio of the partial pressure of water to the partial pressure of chlorine is from about 0.01 to about 2, purging with inert gas and reducing with dry hydrogen.
U.S. Pat. No. 4,678,764 describes a method for redispersing noble metal on a zeolite catalyst by contacting with a gas containing 2 to 20 Torr chlorine, 200 to 4000 Torr oxygen and above 0 to 50 Torr water.
U.S. Pat. No. 4,929,576 describes a method for redispersing agglomerated noble metal on a crystalline aluminophosphate molecular sieve by contacting with chlorine, purging, and reducing with dry hydrogen.
U.S. Pat. No. 4,518,708 describes a process for treating a fresh catalyst comprising iridium or platinum and iridium on a support such as alumina, titanium dioxide, zinc oxide, magnesia, thoria, chromia, silica-alumina, alumina-titania, silica-zirconia, alumina-thoria, etc. by contacting at 850.degree.-1000.degree. F. with a gas having an oxygen concentration of 0.1-50 wt. % for 0.1-24 hours, and at least 0.1 wt. % dry hydrogen halide per weight of catalyst per hour at 700.degree.-1000.degree. C., followed by contacting with a reducing agent at 400.degree.-1000.degree. F. for 0.1-24 hours.
Although methods have been described for redispersing agglomerated noble metal on zeolites, aluminophosphate molecular sieves and alumina, attempts to redisperse agglomerated noble metal supported on silica have not met with great success.
An article by Foger, K. and Jaeger, H., "The Effect of Chlorine Treatment on the Dispersion of Platinum Metal Particles Supported on Silica and Gamma-Alumina", Journal of Catalysis 92, 64-78 (1985) describes the rigorous conditions used in an attempt to redisperse platinum on silica as compared with redispersion of platinum on alumina. With Pt/SiO.sub.2, redispersion was achieved only by treatment with over 25% chlorine at a temperature over 500. Kelvin. But under such severe redispersion conditions, an unwanted side effect was also observed, i.e., the treatment resulted in Pt loss from the silica support as PtCl.sub.2.
Because refractory oxides such as amorphous silica are relatively inert, the propensity for agglomeration of noble metals is greater when these are used as supports. This tendency for metal agglomeration along with metal loss and further agglomeration upon applying conventional chlorine methods for metal redispersion has led to an inability to use noble metals supported on low acidity amorphous refractory oxides in many commercial applications.
Accordingly, it is an object of the invention to modify the interaction between a noble metal and an inactive amorphous support so that metal and the support possess a strong affinity to each other.
It is a more specific object of the invention to provide an effective redispersion of agglomerated platinum on silica without metal loss.